The present invention relates to a transmission apparatus having an infinitely variable drive ratio. It has been designed specifically as an improved type of bicycle transmission but may be adapted to more general applications as well.
The derailleur gear changing mechanism, developed long ago, has, with minimal basic change, become the internationally accepted standard for high performance bicycles. Derailleur is a french word meaning literally to "derail", or in this case, to push a bicycle chain from one sprocket wheel to the next larger or smaller sprocket wheel. By doing so, the derailleur enables a bicycler to change gear ratios while riding. The mechanism consists of brackets, springs, tension idlers, alignment arms, and chain lifters connected by cable to shifting levers mounted on the frame. Because of an imperfect chain-line between the front and rear derailleurs, some "gears" or sprocket combinations that theoretically should work well together frequently do not. Derailleur shifting levers are confusing to the average rider and call for him to give up partial control of the handlebars while trying to pedal and search for a desired sprocket combination or "gear". This is a dangerous procedure for the novice rider, and has led to countless injuries.
The nature of a system employing the derailleur mechanism inherently limits the variety and range of working gear ratios of a given bicycle. Derailleurs are typically employed on "ten-speed" bicycles, in combination with two sprocket wheels attached to the pedal crank of the bicycle and a cluster of five smaller sprocket wheels threaded onto the rear axle hub. Ten combinations, having graduated gear ratios, are thus enabled. The relationship between the number of cogs on these sprockets when linked together by a chain determines the difference between the rotational velocities of the pedal crank and rear wheel. A standard 14, 16, 18, 21, and 24 cog combination sprocket wheel mounted on the rear axle, with 42 and 52 cog sprockets on the crank wheel, allows the derailleur to produce ten graduated gear ratios. The increase in power advantage over the entire range of gear combinations (from 14 and 52 cogs to 24 and 42 cogs), however, is only about two to one (100.3 to 47.3 inches in bicycle gear terms). Such a narrow range is mechanically necessitated by the spacial limitations on a bicycle chassis, which limitations are compounded by the physical versatility limitations of typically fit bicycle riders.
Further, spacial and human versatility those limitations constrain a rider's options to a given set of gear combinations for any particular bicycle, each combination corresponding to a different pedal cadence when traveling on a set slope. Since a constant, rythmic cadence is most efficient, riding efficiency is therefore diminished, especially when traveling over a common terrain that has frequently varying slope. Bicycles thus tend to be geared for particular purposes (e.g. touring, racing, cross country, etc.) and many applications require tailor-made gearing. As refined as derailleur systems are today, they are still limited by mechanical compromises and leave much to be desired.
There is, therefore, a need for a bicycle transmission device which enables the rider to smoothly shift from one gear to another over a wide range of gear ratios with a minimum of effort and attention.
It is further desirable for a transmission device to infinitely vary the gear ratio between the pedal crank and the rear wheel. Individuals vary greatly in their riding ability, and a set of gear ratios suitable for one rider may not be suitable for another rider. Also, as aforementioned, bicycles used for special purposes presently require customized gearing in order to obtain the desired set of gear ratios.
The present invention makes use of what may be referred to as a "conical spool" type of transmission to achieve infinitely variable gear ratios. Many prior devices, such as disclosed in U.S. Pat. Nos. 3,906,809 and 1,048,220 have also utilized oppositely oriented cone pulleys which are mechanically coupled together by means such as a belt. In order to change the gear ratio existing between the cone pulleys, the position of the coupling means must be moved along the length of the pulleys. Prior devices have, therefore, almost uniformly resorted to friction belts to couple the rotation of the cone pulleys together. Relying on friction, however, inherently results in mechanical inefficiencies and necessarily increases the chance of slippage as the cone pulleys are called upon to transmit greater amounts of torque.
A better solution replaces the friction belt with a chain which can mechanically engage cogs or similar structures mounted on each cone pulley. Such mechanical engagement, also referred to as "positive gripping" enables optimum performance as it eliminates the inefficiencies that are inherent with friction coupling belts. The difficulty with positive gripping when applied to a cone pulley system, however, is related to pitch. Since cogs on a cone pulley must necessarily be continuous structures in order for the chain to traverse the cone, the pitch (or spacing between the cogs of a cone pulley must change from one end of the cone pulley to the other). Previously known chains, on the other hand, cannot correspondingly vary their pitch. Chains have, therefore, been difficult to employ in conjunction with a cone pulley system. It is an object of the present invention to provide a means for coupling the rotation of two cones with a chain which mechanically engages each cone even as the chain is moved from one end of the cone to the other.
Although various means have been utilized in the prior art to vary the gear ratio, such as sliding the coupling belt or moving the cone pulleys themselves, none have been satisfactory for the special requirements of a bicycle transmission. It is an object of the present invention, therefore, to provide a bicycle transmission having an infinitely variable gear ratio wherein changes in the gear ratio are effected by utilizing a positive mechanical coupling between conical surfaces, thereby enabling a bicycle that exceeds the performance capabilities of the "ten speed".